In digital printing, the ink layer is created by applying ink drops at the printed media (substrate). Typically, a commercial-quality digital printing system prints four inks: Cyan (C), Magenta (M), Yellow (Y) and Black (K) to create colours on a substrate, and employs halftoning. An image file to be printed is, for example, encoded as four values associated with each image pixel, each of these four values corresponding to a respective one of the four print colours (CYMK). The practical meaning of these values is the “nominal coverage”, that is, the percentage of the substrate to be covered at the pixel location concerned. Thus, by way of example, the printing system might receive an image file in which a pixel had nominal coverage values of: C=50%, M=100%, Y=0%, K=0%; 0% values for Y and K means that for the pixel location concerned no Yellow or Black ink is to be printed, 100% for M means that the pixel location is to be covered completely by Magenta ink, and 50% for C means that half of the pixel location is to be covered by the Cyan ink.
The printing system translates the nominal coverage values into corresponding amounts of ink that is to be put on the substrate in the pixel location concerned. However, the required translation is not straightforward as it will depend on printing conditions. Determining an appropriate translation can be considered as two issues:                how much ink needs to be printed to give 100% coverage?        how much ink needs to be printed to give any particular coverage below 100%?        
With regard to the issue of how much ink is needed to give 100% coverage, it can of course be determined experimentally that the printing system needs to print X picoliters of ink to cover fully the full substrate area associated with a pixel. However, if twice as much ink is printed, the area is also fully covered though now with a darker and more saturated shade of the same ink (due to the ink layer becoming thicker). Using excessive amounts of ink can give rise to a number of problems:                Ink “bleeding”: the ink leaks and flows beyond the area it was originally applied to, thus degrading the image quality;        Ink drying: the ink remains wet long time after the printing;        Colour control: the colour depends on the relative coverage of the ink, as well as on ink layer thickness—so, in the same image, the properties of halftone and continuous tone printing are combined. This is incompatible with the conventional halftone-based approach to colour control and results in uncontrollable colour behaviour.        Excessive ink usage.        
These problems give rise to the need of setting a restriction on the amount of applied ink. To this end, a determination is made as to the amount of ink which will result in coverage of the entire area but avoids/the above-noted problems. The nominal coverage value (called the “ink restriction” value) corresponding to this amount of ink, is then used to scale the nominal coverage values defined in the image file, into corrected coverage values. For example, if it is found that the amount of ink corresponding to 75% nominal coverage value provides enough ink to cover the full area, this ink restriction value of 75% is set as the maximum permissible value and all the original nominal coverage values are scaled down accordingly—see FIG. 1 of the accompanying drawings which illustrates a scaling curve (here a straight line) for converting between nominal and corrected coverage values, with 100% nominal coverage scaling to 75%.
Currently the ink restriction value is set either manually according to the results of visual examination, or semi-automatically with the use of density or Chroma/Lightness scales and some heuristics. These solutions are based mostly oil heuristics and experience, are not robust, subjective, and the results are often less than optimal.
With regard to the issue how much ink needs to be printed to give 50% coverage, this is not just the half of the amount of the 100% layer because of the effect called “dot gain”: once an ink drop hits the substrate, it does not stay exactly in shape but spreads around. This effect is taken into account by a process called “linearization” which provides a further coverage-value scaling curve. For example, this curve may say that to achieve 50% coverage on the substrate, the printing system needs to print 25% of the ink used for the ink restriction value; in this case, the printing system software would go over the image file and replace every 50% coverage value with 25%. This correction goes on top of that described above for scaling coverage values to the ink restriction value; indeed, both corrections are typically done using a single combined scaling curve.
It is an object of the present invention to provide an improved method and apparatus for determining the ink restriction value.